1. Field of the Invention
The present invention relates to a combined oil ring for oil control which is installed in pistons such as mounted in internal combustion engines and compressors.
2. Description of the Related Art
The two-piece type combined oil ring described below is known in the technology of the prior art with an expander installed on the inner circumferential side of the oil ring to increase the contact pressure on the oil ring sliding surfaces and improve oil scrape-off performance.
(1) One combined oil ring is constituted as follows: a groove 13A is formed in the inner circumferential surface of an oil ring 1A, and a coil expander 2A formed of steel wire wound in a coil shape is mounted in the groove 13A (see FIGS. 4A and 4B). This type of combined oil ring is generally utilized as a two-piece type combined oil ring.
(2) In another combined oil ring (see Japanese Patent Publication No. 62-8673), a groove is formed in the inner circumferential surface of an oil ring, a U-shaped cross section expander is installed in the groove, and the legs of the U-shaped cross section make pressing contact with the oil ring groove.
(3) In still another combined oil ring (see Japanese Utility Model Publication No. 47-31927), a groove is formed in the inner circumferential surface of an oil ring, an expander with a U-shaped or V-shaped (turned 90 degrees) or semi-circular shaped cross section is installed in the groove, and an outward protruding portion of the expander cross section makes pressing contact with the oil ring groove.
Recently, increasing demands are being made to make pistons lighter in weight and make piston rings with a smaller axial dimension to attain higher performance in terms of reduced friction and reduced lubricating oil consumption in automobile engines. Oil rings in particular have an expander installed in their inner circumferential side to maintain sufficient tension on the cylinder bore, so that compared to compression rings, oil rings usually have a larger axial width of 3.0 to 4.0 mm, and thus require a smaller axial dimension.
However, in the two-piece type combined oil ring with a coil expander or U-shaped cross section expander of the prior art, making the expander smaller in the axial dimension is difficult. Also, when made axially smaller, an adequate tension is difficult to achieve. The axial dimension of oil rings utilizing such expanders is limited to approximately 2.0 mm for oil rings with coil expanders and approximately 2.5 mm for oil rings with U-shaped cross section expander.
In view of the circumstances of the related art, the present invention therefore has the object of providing a two-piece type combined oil ring smaller in the axial dimension and having superior oil scrape-off performance.
To resolve the problems with the related art, a two-piece type combined oil ring of the present invention comprises an oil ring, and an expander installed in a groove formed on the inner circumferential surface of the oil ring to force the oil ring radially outwards, wherein the expander is formed of ring-shaped plate material corrugated in the axial direction.
The expander is formed of ring-shaped plate material corrugated in the axial direction, so that even when made axially smaller, the expander still has adequate spring force and is capable of attaining high tension. An oil ring with a smaller axial dimension is effective in reducing the piston length and making the piston lighter and more compact. Making the axial dimension smaller also increases the ring flexibility, improves the conformability with the cylinder bore and reduces consumption of lubricating oil. Even when made smaller in the axial dimension, a high tension can be obtained from the axially corrugated expander, so that the consumption of lubricating oil can be reduced even further. The expander is installed within the oil ring groove, so that movement of the expander is restricted and breakage is prevented.
There are no particular restrictions on the cross section of the groove formed on the inner circumferential surface of the oil ring, however, the groove may for instance be formed in a rectangular or trapezoidal shaped cross section. The expander preferably makes pressing contact with the bottom of the oil ring groove in order to obtain the tension which is stable and without variations.
The curvature radius on the corner of the expander which faces the corner at the bottom of the groove is preferably larger than the curvature radius of the corner at the bottom of the groove formed in the inner circumferential surface of the oil ring. The larger curvature radius is preferable for the expander to make pressing contact with the bottom of the oil ring groove stably.
The axial width of the oil ring is preferably 1.0 to 3.0 mm, and more preferably 1.0 to 2.0 mm in order to attain a smaller axial dimension.
The curvature radius of the corner in the bottom of the groove formed in the inner circumferential surface of the oil ring, and the curvature radius of the corresponding opposite corner of the expander are preferably each 0.1 to 1.0 mm in order to obtain the oil ring with a smaller axial dimension.
When the cross section of the groove of the oil ring is for example a rectangular cross section, the upper and lower surfaces of the groove are preferably formed horizontally, and the corresponding opposite upper and lower surface portions of the expander are also preferably formed horizontally. When the cross section of the groove of the oil ring is for example a trapezoidal cross section, the upper and lower surfaces of the groove are preferably formed as tapered surfaces widening towards the inner circumference, and the corresponding opposite upper and lower surface portions of the expander are also preferably formed as tapered surfaces widening towards the inner circumference. Forming the upper and lower surfaces as tapered surfaces in this way is designed to increase the strength of the upper section and the lower section of the oil ring groove.
The oil ring usually has one or two rails forming the outer circumferential sliding surfaces.
Preferably, the oil ring is nitrided or formed with a physical vapor deposition film on the outer circumferential sliding surfaces after nitriding. Also, the expander is preferably nitrided or subjected to electroless nickel plating or chromium plating. The above surface treatments improve the resistance to wear and also maintain satisfactory performance in lubricating oil consumption, etc. for a long period of time. Performing heat treatment such as nitriding prevents a decline in the tension due to rising temperatures during engine operation.
The difference in diameter (roundness) of the expander between the butt ends direction and the butt ends right-angle direction is preferably within 5 mm. The above roundness allows the expander to fit tightly in the oil ring groove, and prevents problems such as slipping or catching due to the expander sticking out of the groove, during installing in the piston or insertion into the cylinder.